support decorations from lookandfeel

Summary:
the Lookand Feel kcm already applies decorations to kwin, allow to
read them as defaults for the case of distribution customization
where a different lnf with a different deco is wanted

Test Plan: tested to start kwin with a different lnf which loads an aurorae decoration

Reviewers: #plasma, #kwin, davidedmundson

Reviewed By: #plasma, #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18261